This invention relates to fuel and oxidant injection into combustion chambers. The invention is particularly applicable for providing hot gas flow for magnetohydrodynamic devices or other applications where it is desirable to provide hot gases substantially free of contaminating slag and other mineral materials. The invention also has application in back-fitting existing combustion units for use with particulate coal or other particulate or highly viscous carbonaceous material. It is contemplated that such feed material can be provided in a liquid or gaseous carrier to the fuel injection system of this invention.
The combustion of carbonaceous material in the form of solid particulates, slurrys of solid particulates or of heavy viscous liquids present considerable problems. Mineral matter within the fuel becomes molten within combustion chambers forming a molten slag that must be removed. Further, in certain applications such as providing hot gases for magnetohydrodynamic (MHD) devices, slag contamination in the electrically conductive seed material prevents its recycle without difficult separation steps. In furnaces and boilers originally designed for light oil or gas use, the close spacing of boiler tubes and other components makes it difficult to subsequently convert to the use of particulate coal or other available fuels. In other instances, particulate carbonaceous material is conveyed in a water slurry requiring the evaporation of the carrier and drying of the fuel within the combustion chamber, which without proper mixing and shaping of the combustion zone can result in quenching or incomplete combustion of fuel materials.
With the recent emphasis on discharge of clean gases from combustion units, the removal of oxides of nitrogen and oxides of sulfur becomes of considerable interest in production processes. Often oxides of nitrogen are minimized through use of a substoichiometric combustion region which establishes a reductive atmosphere for the chemical conversion of organically bound nitrogen in the fuel. Such substoichiometric combustion also contributes to the production of soot having a high carbon content and low porosity. This increases the difficulties in obtaining complete fuel combustion.
In view of the above, it is an object of the present invention to provide an improved fuel injection system for maintaining a stable and symmetrical combustion plume within a combustion chamber.
It is a further object of the invention to provide a fuel and oxidant injection system that permits initial substoichiometric combustion for minimizing oxides of nitrogen emission.
It is also an object to provide a fuel injection system for particulate carbonaceous material that permits adjustment of combustion plume shape within the combustion chamber.
It is one other object of the present invention to provide a method of fuel and oxygen injection that permits good mixing of oxidant and fuel within the combustion chamber and recirculation of hot combustion gases into contact with initially injected gases.
In accordance with the present invention, a fuel injection assembly for use in combination with a combustion chamber is provided. The assembly includes a tubular housing axially mounted onto the combustion chamber. The housing has a throat section near its point of connection with the combustion chamber and accepts an axially adjustable pintle to define an annular chamber therebetween. Axial adjustment of the pintle serves to constrict or enlarge the throat opening into the combustion chamber. The pintle includes an axial passageway for discharging either the fuel or oxidant flows into the combustion chamber. In cooperation with the flow through the pintle, there is provided means for injecting and swirling the second of the oxidant or fuel flows into the annular chamber. The discharges are such as to provide generally concentric impinging flows of fuel and oxidant into the combustion chamber. Adjustment of the pintle respecting the throat section thus constricts or enlarges the opening to shape the combustion region within the chamber.
In more specific aspects of the invention, the carbonaceous fuel is injected in the axial pintle passageway while the oxidant gas is injected through the annular chamber defined between the pintle and tubular chamber.
In further aspects of the invention, a tapered shape is provided inwardly towards the combustion chamber in the throat section and the adjacent mating portion of the pintle resulting in a conical annular passage, whose length and height are variable on axial adjustment of the pintle.
In another specific aspect of the fuel injection assembly, the oxidant swirl means includes a port for admitting oxidant flow into the annular chamber and a ring disposed in the annular chamber between that entry port and the throat section. The ring defines a plurality of passageways through its thickness aligned with both axial and tangential directional components to induce a tangential swirling component to the oxidant flow.
In one other specific aspect, control means are provided for independently varying the oxidant flow to maintain it generally at a constant rate while axially adjusting the pintle.
In other particular aspects, the fuel injection assembly is employed in axial alignment with a combustion chamber at its one end portion. The combustion chamber includes at its opposite end portion an axial opening for discharging combustion gases, a radial opening for discharging molten slag and a generally frustal septum around the axial opening separating it from the radial opening. The combustion chamber may also be connected in communication with an MHD device for the production of electricity or in communication with another utilization device that benefits from generally slag-free combustion gases.
The present invention further contemplates a fuel injection system for the stable combustion of particulate carbonaceous fuels in a combustion chamber from which molten slag is removed. Initial substoichiometric combustion is used to reduce organically bound nitrogen, but yet to minimize soot formation. The system includes means for oxidant injection that provides a flow of oxidant with axial, tangential and radial directional components into the combustion chamber. A pintle is provided for injecting the particulate fuel into the combustion chamber generally concentric within the flow of oxidant.
An important aspect of the invention is the provision of adjustment means for apportioning the driving pressure drop within the oxidant injection means between the tangential directional components and the radial directional components of the oxidant flow. This apportionment permits adjustment of combustion plume shape sufficient to maintain stable combustion while permitting transfer of char and molten slag to the outer boundaries of the plume and generally retaining the combustion gases in the center section of the combustion plume.
In its more specific aspects, the fuel injection system includes a tapered throat section communicating with the combustion chamber and an axially adjustable pintle having a tapered end portion aligned within the tapered throat section such that an annular and generally conical passageway is defined therebetween for oxidant injection into the combustion chamber. This generally conical annular passage provides a radial component to the oxidant injection flow. An axial passageway within the pintle permits the injection of particulate fuel generally concentric within the oxidant flow. Means for swirling the oxidant contributes a tangential directional component such that on axial pintle adjustment an appointment is made of the driving pressure between the tangential and radial directional flow components of the oxidant injection. This permits adjustment of the flow conditions within the combustion plume to provide good mixing of oxidant and fuel as well as proper shaping of the plume in respect to char and slag transfer to the outer boundaries of the combustion chamber.
In yet more specific aspects of the invention, the oxidant flow is provided at 35 to 45% of that required for stoichiometric combustion of the fuel to minimize production of both soot and oxides of nitrogen.
The invention further contemplates a method of injecting oxidant and fuel into a combustion chamber by admitting one of the fluids through an axial passageway of relatively small diameter and the second fluid concentrically around the first fluid. The second fluid includes axial, tangential and radial directional components as it enters the combustion chamber. The shape of the resulting combustion plume within the combustion chamber is varied by apportioning the relative pressure drop between driving pressures providing tangential and driving pressures providing radial flow components within the outer concentric flow. In the specifically contemplated embodiment, the inner fluid is that of particulate carbonaceous fuel material and the outer fluid is an oxidant in a substoichiometric amount respecting the fuel in the concentric flow.
In further aspects of the invention, the relative tangential and radial components of the oxidant flow is varied to effectively shape the combustion plume within the combustion chamber for recirculation of hot combustion gases into contact with the initially injected gases whereby stable and symmetrical combustion dynamics is achieved. In addition, adjustment of these directional components provides for the initial substoichiometric combustion generally near and about the combustion chamber access. It also functions to regulate char transfer to the peripheral boundary of the combustion chamber generally coincidence with the drying and pyrolysis of the carbonaceous particulates.
In other specific aspects, a secondary flow of oxidant is injected tangentially in a direction to supplement the initial tangential directional components. The secondary injection is made at the peripheral boundary of the combustion chamber at no less than 2 and no more than 6 equally spaced locations. This secondary oxidant can be made sufficient to provide stoichiometric and complete combustion of the fuel while driving solid and molten combustion residuals axially along the peripheral boundary layer of the combustion chamber away from the fuel injection point. Molten slag then can be withdrawn from the combustion chamber at the peripheral boundary layer at a location axially removed from the fuel and oxidant injection.